Add new mobile_app webhook command: get_zones (#22604)

## Description:

Adds a new `mobile_app` webhook command, `get_zones`, which just returns all zones.

## Checklist:
  - [x] The code change is tested and works locally.
  - [x] Local tests pass with `tox`. **Your PR cannot be merged unless tests pass**
  - [x] There is no commented out code in this PR.

tests/components/mobile_app/test_webhook.py

@@ -4,8 +4,10 @@ import logging
 import pytest
 
 from homeassistant.components.mobile_app.const import CONF_SECRET
+from homeassistant.components.zone import DOMAIN as ZONE_DOMAIN
 from homeassistant.const import CONF_WEBHOOK_ID
 from homeassistant.core import callback
+from homeassistant.setup import async_setup_component
 
 from tests.common import async_mock_service
 
@@ -100,6 +102,30 @@ async def test_webhook_update_registration(webhook_client, hass_client):  # noqa
     assert CONF_SECRET not in update_json
 
 
+async def test_webhook_handle_get_zones(hass, create_registrations,  # noqa: F401, F811, E501
+                                        webhook_client):  # noqa: F811
+    """Test that we can get zones properly."""
+    await async_setup_component(hass, ZONE_DOMAIN, {
+        ZONE_DOMAIN: {
+            'name': 'test',
+            'latitude': 32.880837,
+            'longitude': -117.237561,
+            'radius': 250,
+        }
+    })
+
+    resp = await webhook_client.post(
+        '/api/webhook/{}'.format(create_registrations[1]['webhook_id']),
+        json={'type': 'get_zones'}
+    )
+
+    assert resp.status == 200
+
+    json = await resp.json()
+    assert len(json) == 1
+    assert json[0]['entity_id'] == 'zone.home'
+
+
 async def test_webhook_returns_error_incorrect_json(webhook_client,  # noqa: F401, F811, E501
                                                     create_registrations,  # noqa: F401, F811, E501
                                                     caplog):  # noqa: E501 F811